How to append to a file in Node

Question

I am trying to append a string to a log file  However writeFile will erase the content each time before writing the string   fs writeFile  log txt    Hello Node   function  err      if  err  throw err    console log  It  s saved              gt  message txt erased  contains only  Hello Node    Any idea how to do this the easy way

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Using fs appendFile or fsPromises appendFile are the fastest and the most robust options when you need to append something to a file    In contrast to some of the answers suggested  if the file path is supplied to the appendFile function   It actually closes by itself  Only when you pass in a filehandle that you get by something like fs open   you have to take care of closing it    I tried it with over 50 000 lines in a file   Examples     async      gt         using appendFile    const fsp   require  fs   promises    await fsp appendFile        path to file     r nHello world              using apickfs  handles error and edge cases better    const apickFileStorage   require  apickfs      await apickFileStorage writeLines        path to directory     filename    Hello world                  Ref  https   github com nodejs node issues 7560  Example Execution  https   github com apickjs apickFS blob master writeLines js

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Node js 0 8 has fs appendFile   fs appendFile  message txt    data to append    err    gt      if  err  throw err    console log  The  data to append  was appended to file           Documentation

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I offer this suggestion only because control over open flags is sometimes useful  for example  you may want to truncate it an existing file first and then append a series of writes to it - in which case use the  w  flag when opening the file and don t close it until all the writes are done  Of course appendFile may be what you re after  -     fs open  log txt    a   function err  log        if  err  throw err      fs writeFile log   Hello Node   function  err          if  err  throw err        fs close log  function err            if  err  throw err          console log  It  s saved

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Your code using createWriteStream creates a file descriptor for every write  log end is better because it asks node to close immediately after the write   var fs   require  fs    var logStream   fs createWriteStream  log txt    flags   a        use  flags   a   to append and  flags   w   to erase and write a new file logStream write  Initial line       logStream end  this is the end line

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When you want to write in a log file  i e  appending data to the end of a file  never use appendFile  appendFile opens a file handle for each piece of data you add to your file  after a while you get a beautiful EMFILE error   I can add that appendFile is not easier to use than a WriteStream   Example with appendFile   console log new Date   toISOString         Array 10000   forEach  function  item index        fs appendFile  append txt   index    n   function  err            if  err  console log err               console log new Date   toISOString       Up to 8000 on my computer  you can append data to the file  then you obtain this     Error  EMFILE  too many open files  open  C  mypath append txt      at Error  native    errno  -4066    code   EMFILE     syscall   open     path   C   mypath  append txt      Moreover  appendFile will write when it is enabled  so your logs will not be written by timestamp  You can test with example  set 1000 in place of 100000  order will be random  depends on access to file   If you want to append to a file  you must use a writable stream like this   var stream   fs createWriteStream  append txt    flags  a     console log new Date   toISOString         Array 10000   forEach  function  item index        stream write index     n        console log new Date   toISOString     stream end      You end it when you want  You are not even required to use stream end    default option is AutoClose true  so your file will end when your process ends and you avoid opening too many files

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Here s a full script  Fill in your file names and run it and it should work  Here s a video tutorial on the logic behind the script    var fs   require  fs     function ReadAppend file  appendFile     fs readFile appendFile  function  err  data        if  err  throw err      console log  File was read         fs appendFile file  data  function  err          if  err  throw err        console log  The  data to append  was appended to file                         edit this with your file names file    name of main file csv   appendFile    name of second file to combine csv   ReadAppend file  appendFile

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Use a  flag to append and create a file  if doesn t exist   fs writeFile  log txt    Hello Node     flag   quot a  quot      err    gt      if  err  throw err    console log  The file is created if not existing            Docs  https   nodejs org api fs html fs file system flags

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In addition to denysonique s answer  sometimes asynchronous type of appendFile and other async methods in NodeJS are used where promise returns instead of callback passing  To do it you need to wrap the function with promisify HOF or import async functions from promises namespace   const   appendFile     require  fs   promises   await appendFile  path to file to append   dataToAppend  optionalOptions     I hope it ll help

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Besides appendFile  you can also pass a flag in writeFile to append data to an existing file   fs writeFile  log txt    Hello Node      flag   a     function err        if  err            return console error err               By passing flag  a   data will be appended at the end of the file

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Try to use flags   a  to append data to a file  var stream   fs createWriteStream  quot udp-stream log quot     flags    a       stream once  open   function fd        stream write msg  quot  r n quot

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My approach is rather special  I basically use the WriteStream solution but without actually  closing  the fd by using stream end    Instead I use cork uncork  This got the benefit of low RAM usage  if that matters to anyone  and I believe it s more safe to use for logging recording  my original use case      Following is a pretty simple example  Notice I just added a pseudo for loop for showcase -- in production code I am waiting for websocket messages   var stream   fs createWriteStream  log txt    flags  a     for true      stream cork      stream write  some content to log      process nextTick      gt  stream uncork         uncork will flush the data to the file in the next tick   In my scenario there are peaks of up to  200 writes per second in various sizes  During night time however only a handful writes per minute are needed  The code is working super reliable even during peak times

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For occasional appends  you can use appendFile  which creates a new file handle each time it s called   Asynchronously   const fs   require  fs     fs appendFile  message txt    data to append   function  err      if  err  throw err    console log  Saved           Synchronously   const fs   require  fs     fs appendFileSync  message txt    data to append      But if you append repeatedly to the same file  it s much better to reuse the file handle

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const inovioLogger    logger         gt        const log file   fs createWriteStream   dirname           inoviopay-  new Date   toISOString   slice 0  10   log     flags   a          const log stdout   process stdout      log file write logger     n

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Using jfile package     myFile text    nThis is new line to be appended     myFile new JFile path

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fd   fs openSync path join process cwd     log txt     a   fs writeSync fd   contents to append   fs closeSync fd

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If you want an easy and stress-free way to write logs line by line in a file  then I recommend fs-extra   const os   require  os    const fs   require  fs-extra     const file    logfile txt   const options    flag   a     async function writeToFile text      await fs outputFile file     text   os EOL    options      writeToFile  First line    writeToFile  Second line    writeToFile  Third line    writeToFile  Fourth line    writeToFile  Fifth line      Tested with Node v8 9 4

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You need to open it  then write to it   var fs   require  fs    str    string to append to file   fs open  filepath    a   666  function  e  id       fs write  id   string to append to file   null   utf8   function        fs close id  function          console log  file closed                        Here s a few links that will help explain the parameters    open write close    EDIT  This answer is no longer valid  look into the new fs appendFile method for appending

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I wrapped the async fs appendFile into a Promise-based function  Hope it helps others to see how this would work      append  path  name  data             return new Promise async  resolve  reject    gt                 try                    fs appendFile  path   name   data  async  err    gt                         if   err                             return resolve  path   name                           else                            return reject err                                                              catch  err                     return reject err

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